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Structural,ACconductivity,dielectricbehaviorandmagneticpropertiesofMg-substitutedLiFe5O8powderssynthesizedby...
ArticleinJournalofMaterialsScienceMaterialsinElectronics·May2015
DOI:10.1007/s10854-015-3181-2
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Structural, AC conductivity, dielectric behavior and magneticproperties of Mg-substituted LiFe5O8 powders synthesizedby sol–gel auto-combustion method
M. G. El-Shaarawy1• M. M. Rashad2
• N. M. Shash1•
M. H. Maklad1• F. A. Afifi3
Received: 7 April 2015 / Accepted: 8 May 2015 / Published online: 14 May 2015
� Springer Science+Business Media New York 2015
Abstract Magnesium-substituted lithium ferrite powders
(Li1-xMg2xFe5-xO8 where x = 0.0–0.8) were prepared by
sol–gel auto-combustion method. XRD profiles revealed
that all compositions were single cubic ferrite structure for
the precursors annealed at 800 �C for 2 h. The crystallite
size was found to decrease whereas the lattice parameter
was increased with increasing Mg2? ion content. The mi-
crostructure of the formed powders was Mg concentration
dependent. Increasing the Mg2? ion ratio was found to
inhibit the grain growth of the formed ferrite. The AC
electrical conductivity of Mg2? ion substituted lithium
ferrite samples was raised with increasing the concentra-
tion of Mg2? ions as the result of increasing the hopping of
electrons between Fe2? and Fe3? ions. Meanwhile, di-
electric constant (e0) was diminished with increasing the
frequency of external electric field as well as increasing the
temperature and concentration of Mg2? ions. Alongside,
the formed crystalline magnesium substituted lithium fer-
rite had good magnetic properties. High saturation mag-
netization (78.7 emu/g) was attained by the formed ferrite
sample at Mg2? molar ratio of 0.4 annealed at 800 �C for
2 h.
1 Introduction
Recent developments of lithium ferrite have attracted a lot
of research interest due to advantageous properties such as
high saturation magnetization, low dielectric losses and
excellent square shaped of hysteresis curve, excellent di-
electric properties, high resistivity *108 X cm, lower eddy
current, relatively high Curie temperature (620 �C), high
chemical inertness, thermal stability, high resistivity and
low eddy current losses, low cost and safety. Due to these
unique properties, it promises to be a potential prolonged
application for microwave devices instead of expensive
yttrium iron garnet (YIG) like circulators, phase shifters,
isolators, power transformation in electronics, memory
core, antennas and high-speed digital tapes. Furthermore, it
is exploited for lithium ion batteries, high-density magnetic
recording, magnetic fluids, magneto-caloric refrigeration,
magnetic resonance imaging enhancement, magnetically
guided drug delivery and multilayer chips inductors [1–5].
The properties of lithium ferrite can be tailored by con-
trolling the synthesis process and the suitable substitution
of metal ions. Numerous wet chemical methods have been
applied to synthesize lithium ferrite fine particles including
ball milling [4], sol–gel [6], combustion method [7, 8],
hydrothermal [5, 10, 11], solvothermal [12], aerosol route
[13] co-precipitation [14] and microemulsion [14] tech-
niques. It is claim mentioning that ferrites powders pre-
pared by sol–gel auto-combustion method technique at low
annealing temperature possesses homogeneous mi-
crostructure, fine particle size, narrow size distribution,
minimum particle agglomeration good chemical purity and
improved physical properties [15–18]. Meanwhile, the
origin of magnetic and dielectric properties of lithium
ferrite is regarded to the variety of metal substitution. The
effect of Mn [2], Zn [2], Co [9, 19], Ce [20], Cr [3], Ge
& M. M. Rashad
rashad133@yahoo.com
1 Physics Department, Faculty of Science, Benha University,
Benha, Egypt
2 Central Metallurgical Research and Development Institute
(CMRDI), Helwan, Cairo, Egypt
3 Basic Engineering Science Department, Faculty of
Engineering, Benha University, Benha, Egypt
123
J Mater Sci: Mater Electron (2015) 26:6040–6050
DOI 10.1007/s10854-015-3181-2
[21], Al [22], Ti [23], Cd [24], Mn–Zr [25], Cu–Ni [26] and
Zn–Zr [27] on various structural, microstructure, electrical,
dielectric and magnetic properties has been reported, and a
mutual influence between the structure and the cationic
distribution is recognized. It seems only very little work
has been done so far on the cation distribution of Mg
substituted lithium ferrites [28]. Therefore, in the present
work, we focused on the synthesis of Mg substituted
lithium ferrite using sol–gel auto-combustion method using
urea as a fuel. Our previous studied involved that the ex-
istence of pure lithium ferrite with good magnetic proper-
ties was obtained using urea based on sol–gel reaction
method at different calcined temperatures from 400 to
800 �C for 2 h. Hence, the impact of Mg2? ion substitution
on the crystal structure, microstructure, conductivity, di-
electric and magnetic properties of the new synthesized
Li1-xMg2xFe5-xO8 nanoparticles with 0.0 B x B 0.8 is
extensively studied using different physical approaches in
this work.
2 Experimental procedure
Li1-xMg2xFe5-xO8 with 0.0 B x B 0.8 have been synthe-
sized using sol–gel auto-combustion method based on urea
as a fuel. Chemically grade ferric nitrate (Fe(NO3)3�9H2O),
lithium nitrate (LiNO3�3H2O), magnesium nitrate
(Mg(NO3)2�6H2O) and urea (NH2�CO NH2) were used as
starting materials. The aqueous solution of ferric, lithium
and magnesium nitrates with Fe3?:Li?:Mg2? molar ratios
(5-x:1.5-x:2x) to attain Li1-xMg2xFe5-xO8 [1] was gently
stirred on hot-plate magnetic stirrer for 15 min. followed
with addition of an aqueous solution of urea to the mixtures
with continuous stirring. The pH of the solution was ad-
justed at 7 by adding ammonia solution to form the gel.
The solution was evaporated to 80 �C with constant stirring
until viscous resin was formed and then dried at 100 �Covernight. For the formation of the Mg substituted lithium
ferrite phase, the dry precursors of Li1-xMg2xFe5-xO8 with
0.0 B x B 0.8 were annealed at the rate of 10 �C/min in
static air atmosphere at 800 �C and maintained at the
temperature for annealed time 2 h. The measurements were
carried out in a current of argon atmosphere. The crys-
talline phases presented in the different annealed samples
were distinguished by X-ray diffraction XRD on a Brucker
axis D8 diffractometer using Cu-Ka (k = 1.5406 A) ra-
diation and secondary monochromator in the range 2h from
20 to 70�. The change in ferrites particles morphologies
was adjusted by field emission scanning electron micro-
scope (FE-SEM, QUANTA FEG 250). The ac electrical
conductivity for the investigated samples was performed
over a temperature range from 30 to 200 �C and frequency
range from 20 Hz to 10 MHz using (LCR-8110G)
precision LCR meter. The magnetic properties of the fer-
rites were measured at room temperature using a vibrating
sample magnetometer (400-1 VSM, U.S., Lake Shore Co.,
Ltd., USA) in a maximum applied field of 20 kOe. From
the obtained hysteresis loops, the saturation magnetization
(Ms), remanence magnetization (Mr) and coercivity (Hc)
were determined Fig. 1.
3 Results and discussion
3.1 Crystal structure
Figure 2 shows the powder X-ray diffraction patterns of
Li1-x Mg2x Fe5-x O8 with 0.0 B x B 0.8 powders syn-
thesized by sol–gel auto combustion using urea annealed at
800 �C for 2 h. It was evident that the magnesium substi-
tuted lithium ferrite specimens contain single ferrite phase.
20 30 40 50 60 702ϴ (degree)
Inte
nsity
(a.u
)
X=0.8
X=0.6
X=0.4
X=0.2
X=0.0
Fig. 1 XRD patterns of Li1-2xMgx Fe5O8 with 0.0 B x B 0.8
powders annealed at 800 �C for 2 h
8.30
8.31
8.32
8.33
8.34
8.35
8.36
8.37
8.38
0.0 0.2 0.4 0.6 0.8 1.0Mg content (x)
Lat
tice
cons
tant
a (°
A)
Fig. 2 Dependence of lattice constant versus dopant content of Mg
J Mater Sci: Mater Electron (2015) 26:6040–6050 6041
123
The diffraction pattern of the unsubstituted sample was
exhibited well crystalline cubic LiFe5O8 (JCPDS #
82-1436) phase. The diffraction peaks belonging to (210)
(211) (220) (311) (400) (422) (511) and (400) planes of
lithium ferrite were indexed. The introducing of Mg2? ions
into pure lithium ferrite phase was executed a small shift
towards low 2h diffraction angle. This shift was slightly
increment the lattice parameter. Figure 2 evinces the de-
pendence of the lattice constant on Mg-substituted content.
It is clear that the lattice constant was increased with
augmenting the magnesium content in the range of
0.0 B x B 0.8. It was attributed to the different ionic radii
of Fe3? (0.64�A), Mg2? (0.71�A), and Li1? (0.76�A) with
a spinel-type structure. The Mg2? ions substituted lithium
Table 1 Average crystallite
size, lattice constant and cell
volume for Li1-2xMgxFe5O8
ferrite samples annealed at
800 �C for 2 h
x Average crystallite size (nm) Lattice constant a (�A) Cell volume V (�A3)
0.0 91.5 8.307 573.219
0.2 84.8 8.334 578.861
0.4 83.6 8.346 581.398
0.6 75.5 8.356 583.406
0.8 61.5 8.367 585.791
x=0.0
x=0.2
x=0.4
x=0.6
x=0.8
Fig. 3 FE-SEM images of Li1-2xMgxFe5O8 with 0.0 B x B 0.8 powders annealed at 800 �C for 2 h
6042 J Mater Sci: Mater Electron (2015) 26:6040–6050
123
ferrite with larger size compared with Fe3? will induce
uniform strain in the lattice as the material is elastically
deformed [28]. This effect associated with the lattice plane
spacing to change and the diffraction peaks shift to a lower
2h position. Noticeably, lattice parameter (a) as well as unit
cell volume (Vcell) of the lithium ferrite specimens was
calculated and the results were listed in Table 1. The
measured lattice parameter for Li1-xMg 2xFe5-xO8 phase
was found to be 0.833 ± 0.01 nm which was in good
agreement with that mentioned in the JCPDS card.
Alongside, Table 1 also records the crystallite sizes of
Li1-x Mg2x Fe5-x O8 ferrites for the most intense peak
[(3 1 1) plane] observed at 2h & 35.825� based on the
XRD data using the Debye–Scherrer formula. It was found
that the crystallite size was minified with addition of Mg2?
ion. It was found to reduce from 91.5 nm for pure lithium
ferrite to 61.5 nm with Mg2? ion substituted molar ratio
0.8.
3.2 Microstructure
Figure 3 displays the field emission scanning electron mi-
croscope (FE-SEM) of Li1-x Mg2x Fe5-xO8 with different
Mg2? ion content 0.0 B x B 0.8 annealed at 800 �C for
2 h. It is clear that the distribution of the grains with uni-
form size in each compositions was observed. Besides, the
grain size was significantly depended on the Mg-substi-
tuted content. The grain size was gradually minimized with
X=0.0
0.5
1.0
1.5
2.0
2.5
3.0
12 12.5 13 13.5 14 14.5 15 15.5 16 16.5
log frequency
log
ε'
30 °C 50 °C 75 °C
100 °C 150 °C 200 °C
X=0.2
1.0
1.5
2.0
2.5
3.0
log frequency
30 °C 50 °C 75 °C
100 °C 150 °C 200 °C
X=0.4
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
log frequency
30 °C 50 °C 75 °C
100 °C 150 °C 200 °C
X=0.6
1.0
1.5
2.0
2.5
3.0
3.5
log frequency
30 °C 50 °C 75 °C100 °C 150 °C 200 °C
X=0.8
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
log frequency
30 °C 50 °C 75 °C100 °C 150 °C 200 °C
log
ε'lo
g ε'
log
ε'lo
g ε'
12 12.5 13 13.5 14 14.5 15 15.5 16 16.5
12 12.5 13 13.5 14 14.5 15 15.5 16 16.5
12 12.5 13 13.5 14 14.5 15 15.5 16 16.5
12 12.5 13 13.5 14 14.5 15 15.5 16 16.5
Fig. 4 Frequency dependence of real part of dielectric constant e0 of Li1-2xMgxFe5O8 with 0.0 B x B 0.8 powders annealed at 800 �C for 2 h
J Mater Sci: Mater Electron (2015) 26:6040–6050 6043
123
increasing Mg-substituted content. The grain size value
was varied from 200 to 500 nm for pure lithium ferrite
to * 60 nm for lithium ferrite substituted by Mg2? ion
molar ratio 0.8. Hence, it can be confirmed that addition
Mg2? ion to lithium ferrite was inhibited the grain growth
of the formed ferrite. The change in the grain size was
attributed to the Mg2? ions (0.71�A) which are substituted
Li? ions (0.76�A) occupy tetrahedral site (A) in the spinel
lattice with a corresponding migration of Fe3? ions
(0.64�A) from A-site to B-site which equilibrates the stress
on B-site causing the observed change in the microstruc-
ture. Meanwhile, the formed particles was transformed
from a quasi-octahedral honey network-like structure to
grain patterns exhibited spherical-like structure with in-
creasing the Mg content.
3.3 Dielectric properties
3.3.1 The frequency dependence of dielectric constant (e0)
Figure 4 indicates the dielectric measurements of the
samples over a temperature range from 30 to 200 �C and
frequency range from 20 Hz to 10 MHz. The results re-
vealed that the dielectric constant (e0) was minified with
X=0.0
0
200
400
600
800
1000
1200
T (°C)
diel
ectri
c co
nsta
nt (ε
')
20Hz
1KHz
10KHz
100KHz
500KHz
1MHz
7MHz
10MHz
X=0.2
0
500
1000
1500
2000
2500
3000
20Hz
1KHz
10KHz
100KHz
500KHz
1MHz
7 MHz
10MHz
X=0.4
0
500
1000
1500
2000
2500
3000
3500
4000
20Hz
1KHz
10KHz
100KHz
500KHz
1MHz
7MHz
10MHz
X=0.6
0
1000
2000
3000
4000
5000
6000
7000
8000
10 35 60 85 110 135 160 185 210 235 260 285
20Hz
1KHz
10KHz
100KHz
500KHz
1MHz
7MHz
10MHz
X=0.8
0
2000
4000
6000
8000
10000
1200020Hz
1KHz
10KHz
100KHz
500KHz
1MHz
7 MHz
10MHz
diel
ectri
c co
nsta
nt (ε
')di
elec
tric
cons
tant
(ε')
diel
ectri
c co
nsta
nt (ε
')di
elec
tric
cons
tant
(ε')
T (°C)10 35 60 85 110 135 160 185 210 235 260 285
T (°C)10 35 60 85 110 135 160 185 210 235 260 285
T (°C)10 35 60 85 110 135 160 185 210 235 260 285
T (°C)10 35 60 85 110 135 160 185 210 235 260 285
Fig. 5 Temperature dependence of real part of dielectric constant e0 of Li1-2xMgxFe5O8 with 0.0 B x B 0.8 powders annealed at 800 �C for 2 h
6044 J Mater Sci: Mater Electron (2015) 26:6040–6050
123
enlarging the frequency. All samples were exhibited the
similar frequency dependence of e0. The variation in e0 with
frequency can be understood on the basis of Maxwell and
Wagner two layer model in accordance with Koop’s phe-
nomenological theory [29]. The dielectric structure of
ferrites is made up of well conducting layer of grains fol-
lowed by poorly conducting layer of grain boundaries and
the high value of dielectric constant arises from the space
charge polarization produced at the grain boundary. The
polarization mechanism involves the exchange of electrons
between the ions of the same element, which are present in
more than one valence state and are distributed randomly
over crystallographic equivalent sites. Here the exchange
of electrons mainly takes place between Fe3? and Fe2?
ions present at octahedral sites (B-site). During this ex-
change mechanism, the electrons have to pass through the
grains and grain boundary of the dielectric medium. Owing
to high resistance of the grain boundary, the electrons ac-
cumulate at the grain boundary and produce space charge
polarization. Furthermore, it can be seen that the dielectric
constant was decreased rapidly in the low frequency region
and became frequency independent in the high frequency
region. It is well known that the grain boundaries are more
effective in low frequency region and the grains are ef-
fective in the high frequency region [29]. Therefore, due to
the grain boundary effect, the dielectric constant decreases
rapidly in the low frequency region. In the high frequency
region, the grains come into action and also the hopping of
electrons cannot follow the high frequency ac field,
therefore, the dielectric constant decreases and becomes
frequency independent.
3.3.2 The temperature dependence of dielectric constant
(e0)
Figure 5 describes the variation of the dielectric constant e0
with temperature for all investigated samples of Li1-x
Mg2x Fe5-x O8 with 0.0 B x B 0.8 powders annealed at
800 �C for 2 h over a temperature range from 30 to 200 �Cand frequency range from 20 Hz to 10 MHz. The data
showed that dielectric constant e0 was nearly constant with
low temperature and it was slightly changed with the fre-
quency. This behavior can be ascribed to the partially
electronic polarization, which played a significant role in
this region. Otherwise, a lightly increase in e0 was observed
with raise the temperature. This was attributed to the ionic
polarization that beginning to play a role in cooperation
with the electronic one. Consequently, the participation of
more than one type of polarization led to increasing the
value e0 with temperature. Hence, it can conclude that the
dielectric constant is almost temperature independent due
to the dipole remained frozen at the relatively low tem-
perature range. Further, more than one type of polarization
of the dipoles was expected at relatively high temperature
by increasing temperature. Subsequently, the dipoles rota-
tions became easier and thus e0 was increased with
temperature.
3.3.3 Effect of Mg concentration on dielectric constant (e0)
Figure 6 elucidates the effect of Mg concentration (x) on
dielectric constant at stable temperature 200 �C. It is illu-
minated that e0 was increased with increasing the concen-
tration of Mg2? ions. It is known that the Li? ions occupy
only the octahedral site (B-site) in the spinel lattice
whereas the Mg2? ions prefer to occupy both the tetrahe-
dral and octahedral sites [28, 30]. Therefore, with the in-
crease in the concentration of Mg2? ions, some of the
Mg2? ions occupy the tetrahedral site (A-site) and so
displace the Fe3? ions from A-site to B-site. Thus more
Fe2?–Fe3? ion pairs are available at B-site for hopping
mechanism which results in the increase in dielectric po-
larization and hence of the dielectric constant.
3.4 Electrical properties
3.4.1 The temperature dependence of AC conductivity
Figure 7 illustrates the temperature dependence of AC
conductivity, rac, for all investigated samples of Li1-x
Mg2xFe5-xO8 with 0.0 B x B 0.8 powders annealed at
800 �C for 2 h over a temperature range from 30 to 200 �Cand frequency range from 20 Hz to 10 MHz. The relation
between the temperature and AC conductivity rac was
estimated from the Arrhenius expression as the following:
rac ¼ ro exp �EaðacÞ= kT� �
ð1Þ
where ro is the pre- exponential factor contains several
constants, k is the Boltzmann’s constant, T is the absolute
T=200 °C
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
12 12.5 13 13.5 14 14.5 15 15.5 16 16.5
log frequency
log
ε'
X=0.0 X=0.2 X=0.4X=0.6 X=0.8
Fig. 6 The dependence of real part of dielectric constant e0 on Mg
content (x) of Li1-2xMgxFe5O8 with 0.0 B x B 0.8 powders annealed
at 800 �C for 2 h
J Mater Sci: Mater Electron (2015) 26:6040–6050 6045
123
temperature and Ea(ac) is the AC activation energy for
conduction. The activation energies were calculated from
straight line fit using Eq. 1 compositions and they were
given in Table 2. It is noticeable that the AC conductivity
rac was steadily increased upon increasing of temperature
and Mg2 ? ion content. However, the mechanism of
electrical conduction is similar to that of dielectric polar-
ization. In the low frequency region, grain boundaries with
X=0.0
-18
-16
-14
-12
-10
-8
-6
-4
-2
2 2.25 2.5 2.75 3 3.25 3.5 3.75 4
1000 / T (K ¹)
log
ac c
ondu
ctiv
ity
20Hz
1KHz
10KHz
100KHz
500KHz
1MHz
7MHz
10MHz
X=0.2
-14
-12
-10
-8
-6
-4
-2
0
2 2.25 2.5 2.75 3 3.25 3.5 3.75 4
1000 / T (K ¹)
log
ac c
ondu
ctiv
ity
20 Hz
1 KHz
10 KHz
100 KHz
500 KHz
1 MHz
7 MHz
10 MHz
X=0.4
-14
-12
-10
-8
-6
-4
-2
0
2
2 2.25 2.5 2.75 3 3.25 3.5 3.75 4
1000 / T (K ¹)
log
ac c
ondu
ctiv
ity
20Hz
1KHz
10KHz
100KHz
500KHz
1MHz
7MHz
10MHz
X=0.6
-14
-12
-10
-8
-6
-4
-2
0
2
2 2.25 2.5 2.75 3 3.25 3.5 3.75 4
1000 / T (K ¹)
log
ac c
ondu
ctiv
ity
20Hz
1KHz
10KHz
100KHz
500KHz
1MHz
7MHz
10MHz
X=0.8
-12
-10
-8
-6
-4
-2
0
2
2 2.25 2.5 2.75 3 3.25 3.5 3.75 4
1000 / T (K ¹)
log
ac c
ondu
ctiv
ity
20Hz
1KHz
10KHz
100KHz
500KHz
1MHz
7MHz
10MHz
Fig. 7 Electrical conductivity versus inverse of temperatures for Li1-2xMgxFe5O8 with 0.0 B x B 0.8 powders annealed at 800 �C for 2 h
6046 J Mater Sci: Mater Electron (2015) 26:6040–6050
123
high resistance were effective, giving a constant plateau
region (rac). At higher frequencies, the increase in con-
ductivity was imputed to grain effect and increased hop-
ping of charge carriers between Fe2?–Fe3? ions at the
adjacent octahedral sites [28, 30]. On the other hand, the
increase in conductivity with temperature was related to
thermally activated hopping of charge carriers [28, 30].
Additionally, the increment of rac with Mg2? ions ratio
was attributed to the increase in the magnitude of elec-
tronic exchange which was dependent on the concentration
of Fe3?/Fe2? ion pairs present on B-site as depicted in
Fig. 8.
3.4.2 The frequency dependence of AC conductivity
Generally, the total conductivity is expressed bythe fol-
lowing power relation
rac xð Þ ¼ A xs ð2Þ
where A is a temperature dependent parameter, x is the
angular frequency and the frequency exponent s is a tem-
perature-dependent parameter with values between 0.0 and
1. Figure 9 presents the variation of electrical conductivity
rac (x) with frequency 20 Hz to 10 MHz. One can note
that values of conductivity was gradually increased with
increasing the applied field frequency. The increasing in
(rac) with applied field frequency was explicated in the fact
that the pumping force of the applied frequency that helps
in transferring the charge carriers between the different
localized states as well as liberating the trapped charges
from the different trapping centers. These charge carriers
participate in the conduction process concurrent with
electrons produced from the valence exchange between the
different metal ions. The exponent s was determined as a
function of composition for each samples, in the tem-
perature range from 30 to 200 �C by plotting ln r versus ln
x according to Eq. 2 as depicted in Fig. 9 which repre-
sents straight lines with slope equal to the exponent s and
the variation of the exponent s is given in Table 3. It is
obviously that the value of s was between 0 and 1. When
s = 0, the electrical conduction was frequency independent
or DC conduction, but for s B 1, the conduction was fre-
quency dependent or AC conduction. Consequently, it is
distinctly that the exponent s was located in two ranges. In
the range 0.5 B s B 1 which evidenced the electron hop-
ping between Fe?2 and Fe?3 ions and the values of s B 0.5
which demonstrated the domination of ionic conductivity
[30]. Table 3 devotes the values of s which proved that the
exponent s was positioned in the two ranges. Therefore, the
manner of r0
with frequency and S with temperature de-
noted that the classical barrier hopping model is the most
favorable mechanism to expound the conduction mechan-
ism for the compositions under investigation. The charge
carriers responsible for the conduction process are the
electron hopping between Fe2? and Fe3? ions in the case
of Mg-substituted lithium ferrite samples.
3.5 Magnetic properties
The magnetization of Li1-x Mg2x Fe5-x O8 with
0.0 B x B 0.8 powders was measured at room temperature
under an applied field of 20 kOe and the hysteresis loops of
the ferrite powders were obtained. Figure 10 shows the
nexus of magnetization (M) as a function of applied field
(H). Subsequently, the magnetic parameters are listed in
Table 4. It is distinctly that the saturation magnetization of
the ferrite powders was increased with increasing the Mg2?
content upon x = 0.4. Thereafter, it was decreased with
Table 2 Activation energies
obtained from Arrhenius plots
for Li1-2xMgxFe5O8 ferrite
samples annealed at 800 �C for
2 h
x Ea(ac) (eV)
20 Hz 1 kHz 10 kHz 100 kHz 500 kHz 1 MHz 7 MHz 10 MHz
0.0 0.246 0.183 0.286 0.264 0.521 0.160 0.054 0.073
0.2 0.023 0.092 0.109 0.041 0.028 0.033 0.049 0.041
0.4 0.009 0.036 0.060 0.128 0.188 0.120 0.006 0.049
0.6 0.028 0.039 0.067 0.054 0.022 0.046 0.050 0.215
0.8 0.061 0.045 0.046 0.079 0.215 0.033 0.091 0.104
T=200 °C
-12.0
-10.0
-8.0
-6.0
-4.0
-2.0
8 9 10 11 12 13 14
log frequency
log
ac c
ondu
ctiv
ity
X=0.0 X=0.2 X=0.4X=0.6 X=0.8
Fig. 8 Dependence of ac electrical conductivity on Mg content (x) of
Li1-2xMgxFe5O8 with 0.0 B x B 0.8 powders annealed at 800 �C for
2 h
J Mater Sci: Mater Electron (2015) 26:6040–6050 6047
123
x[ 0.4. The observed magnetization was a result of the
simultaneous influence of several extrinsic and intrinsic
factors such as density, anisotropy, grain size, cation dis-
tribution and the A–B exchange interaction [29–32]. The
saturation magnetization is, therefore, explained by con-
sidering the metal ion distribution and antiparallel spin
alignment of the two sublattice sites. According to Neel’s
model, of the three types of AA, AB and BB interactions,
intersublattice A–B superexchange interaction is the
strongest. The net magnetization is given by the vector sum
of the magnetization of the two sublattices A and B. The
initial increase in Ms may be due to the fact that some of
the Mg2? ions, which are incorporated in place of Li? ions,
occupy tetrahedral site (A) in the spinel lattice with a
corresponding migration of Fe3? ions from A-site to B-site.
This increases the magnetization of the B sublattice while
that of A sublattice decreases as Fe3? ions. With further
incorporation of Mg2? ions (x[ 0.4), the decrease in Ms
can be attributed to the increased migration of Fe3? ions
from the A- to B-site in order to accommodate the Mg2?
ions at the A-site. This migration results in an increase in
Fe3? ion concentration at B-site which gives rise to
X=0.0
-16
-14
-12
-10
-8
-6
-4
log frequency
log
ac c
ondu
ctiv
ity
30 °C 50 °C 75 °C
100 °C 150 °C 200 °C
X=0.2
-12
-10
-8
-6
-4
-2
log frequency
log
ac c
ondu
ctiv
ity
30 °C 50 °C 75 °C
100 °C 150 °C 200 °C
X=0.4
-12
-10
-8
-6
-4
-2
8
log frequency
log
ac c
ondu
ctiv
ity
30 °C 50 °C 75 °C
100 °C 150 °C 200 °C
X=0.6
-10
-9
-8
-7
-6
-5
-4
-3
-2
8 9 10 11 12 13 14 8 9 10 11 12 13 14
log frequency
log
ac c
ondu
ctiv
ity
30 °C 50 °C 75 °C100 °C 150 °C 200 °C
X=0.8
-10
-9
-8
-7
-6
-5
-4
-3
-2
8 9 10 11 12 13 14
9 10 11 12 13 14
8 9 10 11 12 13 14
log frequency
log
ac c
ondu
ctiv
ity
30 °C 50 °C 75 °C100 °C 150 °C 200 °C
Fig. 9 Dependence of AC conductivity on the frequency of Li1-2x Mgx Fe5O8 with 0.0 B x B 0.8 powders annealed at 800 �C for 2 h
6048 J Mater Sci: Mater Electron (2015) 26:6040–6050
123
antiparallel spin coupling and spin canting. This results in
the weakening of the A–B exchange interaction, leading to
the reduction of the magnetization [29–33]. Moreover
anomalous behavior of Hc was due to the different mag-
netic moment of Fe and Mg. This may be imputed to
changes in the particle size and the low magnetic
anisotropy, which arises from crystal imperfection [33, 34].
4 Conclusions
Li1-x Mg2x Fe5-x O8 with 0.0 B x B 0.8 nanopowders
have been successfully synthesized by sol–gel auto-com-
bustion method using urea as an organic fuel. The results
from XRD, FE-SEM, dielectric constant, AC electrical
conductivity and VSM studies are summarized as follows:
• XRD analysis showed that all the samples were formed
in single phase cubic spinel structure.
• The introducing of Mg2? ions into pure lithium ferrite
phase was occurred a small shift towards low 2hdiffraction angle. Thus, the lattice constant was
increased with addition of Mg2? ion.
• The crystallite size of the formed powders was found to
minify with increasing the Mg content. It was found to
decrease from 91.5 nm for pure lithium ferrite to
61.5 nm with Mg2? ion molar ratio of 0.8.
• The microstructure of Li1-x Mg2x Fe5-x O8 with
0.0 B x B 0.8 powders was dependent on concentra-
tion of magnesium. the formed particles was converted
from a quasi-octahedral honey network-like structure to
grain patterns exhibited spherical-like structure with
increasing the Mg content.
• Dielectric constant (e0) was decreased with the enhanc-
ing the frequency of external electric field whereas it
was found to increase with temperature.
• Ac conductivity of the formed ferrite powders was
slightly increased at low frequencies and abruptly at
high frequencies. Moreover, Ac conductivities of these
ferrites were increased as the result of the increment of
temperature and concentration of Mg2? ions due to
increase the hopping of electrons between Fe2? and
Fe3? ions.
• The formed crystalline pure magnesium substituted
lithium ferrite powders had good magnetic properties.
High saturation magnetization (78.7 emu/g) was ful-
filled for the formed ferrite at annealing temperature
800 �C for 2 h with Mg content 0.4. Such ferrite
nanomaterials would be a good candidate for several
potential applications including microwave devices and
high-density magnetic recording media. Meanwhile,
these ferrite compositions are useful for the multilayer
chip inductors which are important components for
electronic products such as notebook computer and
cellular phone [1, 2].
Acknowledgments This research is financially supported by Sci-
ence and Technology Development Fund STDF, Egypt, Grant No.
Project ID 3681.
References
1. M.M. Rashad, M.G. El-Shaarawy, N.M. Shash, M.H. Maklad,
F.A. Afifi, J. Magn. Magn. Mater. 374, 495 (2015)
2. Dipti, P. Kumar, J.K. Juneja, S. Singh, K.K. Raina, C. Prakash,
Ceram. Int. 41, 3293 (2015)
3. M. Srivastava, S. Layek, J. Singh, A.K. Das, H.C. Verma, A.K.
Ojha, N.H. Kim, J.H. Lee, J. Alloys Compd. 591, 174 (2014)
Table 3 The values of the exponent ‘‘s value’’ for Li1-2xMgxFe5O8
ferrite samples annealed at 800 �C for 2 h at different temperatures
x S-value
30 �C 50 �C 75 �C 100 �C 150 �C 200 �C
0.0 0.692 0.648 0.831 0.998 0.874 0.855
0.2 0.798 0.838 0.81 0.673 0.593 0.537
0.4 0.603 0.718 0.645 634 0.351 0.363
0.6 0.362 0.493 0.397 0.262 0.525 0.529
0.8 0.294 0.459 0.252 0.375 0.462 0.427
-100
-80
-60
-40
-20
0
20
40
60
80
100
-25000 -15000 -5000 5000 15000 25000
Field (Oe)
Mag
netiz
atio
n (e
mu/
g)
X=0.0X=0.2X=0.4X=0.6X=0.8
Fig. 10 M-H hysteresis loop of Li1-2xMgxFe5O8 with 0.0 B x B 0.8
powders annealed at 800 �C for 2 h
Table 4 The magnetic properties for Li1-2xMgxFe5O8 ferrite sam-
ples annealed at 800 �C for 2 h
x Ms (emu/g) Mr (emu/g) Hc (Oe)
0.0 51.9 7.96 145.7
0.2 53.6 9.11 162.4
0.4 78.7 2.50 36.0
0.6 34.9 0.62 21.6
0.8 40.3 3.34 97.6
J Mater Sci: Mater Electron (2015) 26:6040–6050 6049
123
4. S.A. Mazen, N.A. Abu-Saad, Appl. Nanosci. 5, 105 (2015)
5. H. Zeng, T. Tao, Y. Wu, W. Qi, C. Kuang, S. Zhou, Y. Chen,
RSC Adv. 4, 23145 (2015)
6. P.V.B. Reddy, B. Ramesh, ChG Reddy, Phys. B 405, 1852 (2010)
7. A. Sutka, G. Mezinskis, Front. Mater. Sci. 6, 128 (2012)
8. R.P. Patil, B.V. Jadhav, P.P. Hankare, Results Phys. 3, 214 (2013)
9. V.S. Sawant, K.Y. Rajpure, J. Magn. Magn. Mater. 382, 152
(2015)
10. X. Wang, L. Gao, L. Li, H. Zheng, Z. Zhang, W. Yu, Y. Qian,
Nanotechnology 16, 2677 (2005)
11. H. Zeng, T. Tao, Y. Wu, W. Qi, C. Kuang, S. Zhouand, Y. Chen,
RSC Adv. 4, 231–245 (2014)
12. B. Li, Y. Xie, H. Su, Y. Qian, X. Liu, Solid State Ion. 120, 251
(1999)
13. S. Singhal, K. Chandra, J. Electromagn. Appl. Anal. 2, 51 (2010)
14. M.A. Dar, J. Shah, W.A. Siddiqui, R.K. Kotnala, J. Alloys
Compd. 523, 36 (2012)
15. M. Rasly, M.M. Rashad, J. Magn. Magn. Mater. 337–338, 58
(2013)
16. M.M. Rashad, A.O. Turky, A.T. Kandil, J. Mater. Sci.: Mater.
Electron. 24, 3284 (2013)
17. M.M. Rashad, J. Mater. Sci.: Mater. Electron. 23, 882 (2012)
18. M.M. Rashad, M.G. Fayed, T.M. Sami, E.E. El-Shereafy, J.
Mater. Sci.: Mater. Electron. 26, 1259 (2015)
19. G. Aravind, M. Raghasudha, D. Ravinder, J. Magn. Magn. Mater.
378, 152 (2015)
20. Viswarupa Mohanty, Rajesh Cheruku, Lakshmi Vijayan, G.
Govindaraj, J. Mater. Sci. Technol. 30, 335 (2014)
21. S.A. Mazen, N.I. Abu-Elsaad, Ceram. Int. 40, 11229 (2014)
22. P.R. Arjunwadkar, R.R. Patil, J. Alloys Compd. 611, 273 (2014)
23. B.K. Kuanr, G.P. Srivastava, J. Appl. Phys. 75, 6115 (1994)
24. T. Namgyal, J. Singh, K. Chandra, S. Bansal, S. Singhal, J. Mol.
Struct. 1019, 103 (2012)
25. M.J. Iqbal, M.I. Haider, Mater. Chem. Phys. 140, 42 (2013)
26. M. Ramesh, G.S.N. Rao, K. Samatha, B.P. Rao, Ceram. Int. 41,
1765 (2015)
27. S.K. Gurav, S.E. Shirsath, R.H. Kadam, S.M. Patange, K.S. Lo-
har, D.R. Mane, Mater. Res. Bull. 48, 3530 (2013)
28. Y.-P. Fu, S.-H. Hu, Ceram. Int. 36, 1311 (2010)
29. M.A. Dar, K.M. Batoo, V. Verma, W.A. Siddiqui, R.K. Kotnala,
J Alloys Compd. 493, 553 (2010)
30. A.A. Kadam, S.S. Shinde, S.P. Yadav, P.S. Patil, K.Y. Rajpure, J.
Magn. Magn. Mater. 329, 59 (2013)
31. R. Cheruku, G. Govindaraj, L. Vijayan, Mater. Chem. Phys. 146,
389 (2014)
32. S.S. Teixeira, M.P.F. Graca, L.C. Costa, M.A. Valente, Mater.
Sci. Eng. B 186, 83 (2014)
33. S.I. Hussein, A.S. Elkady, M.M. Rashad, A.G. Mostafa, R.M.
Megahid, J. Magn. Magn. Mater. 379, 9 (2015)
34. M.M. Rashad, M.I. Nasr, Electron. Mater. Lett. 8, 325 (2012)
6050 J Mater Sci: Mater Electron (2015) 26:6040–6050
123
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